Recently, as electronic devices have been size-reduced, improved in performance and improved in portability, rechargeable secondary batteries such as a Ni-MH alkali storage battery and a lithium secondary battery have been practically and extensively used. In particular, the use of a lightweight lithium-ion secondary battery with a high energy density has been investigated not only for conventional small information-communications devices such as cell phones and laptop computers, but also for moving vehicles such as automobiles, power sources of rotating bodies such as electric tools and backup power sources which are required to have high-output properties and long-term reliability.
Currently, a cathode active material for a lithium-ion secondary battery is a lithium transition metal complex oxide such as lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2) and lithium manganese oxide having a spinel structure (LiMn2O4) which is capable of providing an operating voltage of 4V. Meanwhile, carbon materials are extensively used as an anode active material. Application of a conventional electrode material to industrial applications such as backup system for industrial applications and to an automobile battery which are expected to give rise to great demand, however, has problems to be solved such as drain on resources and a high price, dissatisfactory battery performance and safety.
Recently, a lithium-titanium complex oxide has drawn attention as an electrode active material for a lithium secondary battery used in the above applications. A lithium-titanium complex oxide exhibits a charge-discharge potential nobler than that of a carbon material (a spinel type Li4Ti5O12 has a potential of about 1.56 V to Li/Li+), and is, therefore, practically used as a cathode active material for a battery in a wristwatch. Among lithium-titanium complex oxides, Li4Ti5O12 having a spinel type structure has a large amount of lithium ions which can be electrochemically intercalated or deintercalated and exhibits smaller volume change associated with the intercalation and deintercalation, so that it advantageously maintains a crystal structure and exhibits less deterioration caused by charge-discharge cycle. Furthermore, it is known that since a lithium ion exhibits a nobler intercalation/deintercalation potential than that of a carbon material, precipitation of lithium metal at a low temperature and reductive decomposition of a solvent by an anode active material are prevented, resulting in ensuring of safety and a longer battery life. However, for a lithium-titanium complex oxide, an electron conductivity is extremely small, a reaction resistance is large for intercalation/deintercalation of a lithium ion, and charge/discharge under high load leads to significant deterioration in battery properties. It is difficult, therefore, to apply to a battery system for which a higher output is required.
For solving these problems, there have been proposed homogenization of lithium-titanium complex oxide particles, size reduction of the particles and complexing with an electron conductive substance.
Size reduction in lithium-titanium complex oxide particles can increase an area where a reaction proceeds and reduce intra-particle diffusion length of lithium ions and electrons, but mere size reduction of particles causes poor subsequent precipitation (dispersibility) and poor adherence to a collector in producing an electrode and adversely causes deterioration in battery properties such as a capacity and an output, resulting in insufficient improvement in charge/discharge properties under high load (Non-patent Reference No. 1).
Meanwhile, there has been disclosed, as a method for endowing a lithium-titanium complex oxide with electric conductivity, doping a crystal lattice of a lithium-titanium complex oxide with dissimilar metal, for example, doping lithium titanate with an element having a higher valence than Ti(IV) (V, Nb, Mo, P) (Patent Reference No. 1), and disclosed an active material for a battery wherein transition metal (V, Zr, Nb, Mo, Mn, Fe, Cu, Co) is doped (Patent Reference No. 2). There has been disclosed as a method for coating an active material surface with an electroconductive substance such as carbon, a metal and an oxide, an active material for a battery wherein the surfaces of lithium titanate particles are coated with carbon at a particular ratio (Patent Reference No. 3).
In an electrode reaction, whereas oxidation and reduction of electrodes proceed by discharge and storage of lithium ions and transfer of electrons in the course of charge or discharge, an electrode reaction rate, that is, an output property, depend on a dispersion rate of lithium ions and electron mobility within the solid phase of an active material. The disclosed method for endowing a less electroconductive lithium-titanium complex oxide with electroconductivity fails to adequate electroconductivity due to the absence of a highly conductive substance within the particle and thus a load resistance due to electron transfer is not satisfactorily reduced. In a method wherein the surfaces of particles are coated with carbon by thermal decomposition of an organic compound, the carbon exhibits poor graphite crystallinity and thus poor electroconductivity, and in the coated area, the active material surface is not directly in contact with an electrolyte, and thus spreading diffusion of lithium ions are prevented, which is disadvantageous to, for example, output properties and a utilization rate of an active material.